I. Field of the Invention
The present invention relates to an improved internal thread gage.
II. Description of the Prior Art
It is oftentimes necessary to accurately measure the pitch diameter of an internal thread. There are two previously known and commonly employed methods for measuring internal threads.
One previously known method for measuring internal threads requires the construction of two externally threaded plugs, commonly referred to as "go" and "no-go" plugs. The go plug is carefully machined to the lower end of the internal thread tolerance while, vice versa, the no-plug is carefully machined to the upper end of the internal thread towers. Consequently, if the go plug can be screwed into the internal thread being checked while the no-go plug cannot, the internal thread lies within the specified tolerance range.
The use of go and no-go plugs, however, is disadvantageous in that two separate plugs must be carefully machined in order to check a single internal thread. Although it would be possible to maintain a complete set of go and no-go plugs, conventional threads varying not only in diameter, but also in their pitch, thread size, number of starts and the like. Consequently, an even reasonably complete set of go and no-go plugs would require the manufacture and storage of tens of thousands of such plugs. For this reason, go and no-go plugs are conventionally constructed for a specific thread to be checked which is time consuming, requires special tooling and is, therefore, unduly expensive.
In lieu of the go and no-go plugs, there are previously known thread gages specifically designed to measure the pitch diameter of an internal thread. One such previously known thread gage is described in U.S. Pat. No. 3,827,157, which issued on Aug. 6, 1974, to Cass Kaifesh.
These previously known internal thread gages such as that described in the aforementioned patent conventionally include a body having a pair of arms extending outwardly from the body. A single thread engaging element is formed at the free end of one arm while a pair of spaced thread engaging elements are formed at the free end of the other arm. With the three thread engaging elements properly seated in the internal thread being checked, the pitch diameter is read from an indicator mounted to the body of the internal thread gage.
The disadvantage of this type of previously known internal thread gage, however, is that a new set of arms must be mounted to the gage for each different type of thread; i. e. threads with a different pitch, different number of starts or the like. Otherwise, the three point contact between the gage arms and the internal threads will not properly seat between the internal threads being checked and, therefore, will produce an inaccurate reading on the indicator. As a result, in addition to the gage body itself, it is necessary to obtain a plurality, oftentimes hundreds, of different thread engaging arms in order to measure different types of threads.